Restarting a X11 window manager is a busy process, trying to leniently
quit the main loop may result in old and new instances each having a
frames client up and running, and the window handover to be less clean
than it should due to the frames client that is about to exit still
being able to react to the batch of events resulting from the window
manager switch that is already undergoing.
In order to avoid extending this transition period any long, make
the frames client exit() the process immediately when SIGTERM is
gotten from the parent process.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2796>
a9bf493ec7